Traditionally, multifocal lenses have been produced in glass by fusing a small glass segment, or button, member of high lens power into a countersink on a larger lens blank, commonly referred to as the "major". This practice is described in detail in my U.S. Pat. No. 4,842,632.
It is not practical to employ this procedure in producing organic polymer, multifocal lenses. Hence, such lenses are moulded in a one-piece construction of a single material. One surface may be a smooth continuous surface having a single curvature. The other surface, usually, but not necessarily, the front surface, must be contoured to provide the required corrective curvatures. In a bifocal lens, these are the near and far vision corrections. Necessarily, the corrective curvatures have different radii of curvature. Hence, they are separated by a narrow wall extending between an edge on one surface of corrective curvature and the second corrective surface of curvature.
Currently, organic lenses are produced by filling a cavity with a liquid monomer containing known polymerizing additives. The cavity may be formed by an assembly of two glass mould members held in spaced relationship by a flexible gasket.
The surface contours of the two mould members will be precisely the reverse of the surface contours desired on the organic lens. As a result, the prescription power and surface finish quality of an organic multifocal lens relies entirely on the finished accuracy of the glass mould members against which the lens is formed.
The primary glass mould member, against which the corrective front surface of an organic multifocal lens is formed, is a precision fused assembly contoured to provide the different curvatures required. As described in detail later, one of the fused members in this assembly has, heretofore, been ground and polished, before fusion and finishing, to provide an angular wall in the fused assembly. This has been necessary to permit opening of a glass mould to remove a moulded organic lens without damage to the lens. Customarily, the glass mould members are of an ophthalmic quality, white crown glass, e.g. an alkali-lime-silicate, or alkali-zinc-silicate composition.
Production of the glass mould members is a very specialized, and hence expensive, operation that requires frequent inspections and measurements to achieve the necessary accuracy. Moreover, the polished glass mould members are very prone to damage. Therefore mould life is quite unpredictable, and may vary from one or two castings to as many as a hundred. Hence, constant checking of mould surfaces is a necessity, as is maintaining a substantial inventory of mould members for replacement.